Uncoupling of metabolism to reduce biomass production in the activated sludge process

Production of excess biomass during biological treatment of wastewaters req uires costly disposal. Also with environmental and legislative constraints limiting disposal options, considerable impetus exists for reducing the amo unt of biomass produced. Uncoupling metabolism in activated sludge may redu ce biomass production and this approach is investigated in conjunction with consequences upon substrate removal and population dynamics. To induce unc oupled metabolism, para-nitrophenol (pNP) a known protonphoric uncoupler of oxidative phosphorylation, was introduced to a bench-scale activated sludg e process. Microbial populations were monitored by both microscopic and by three methods of molecular analyses. Presence of the protonphore caused a s hift in the microbial population with protozoa being washed out of the syst em and filamentous bacteria proliferating. The molecular composition of the microbial community was determined by PCR amplification of 16SrRNA genes a nd subsequent denaturing gradient gel electrophoresis (DGGE). Band Patterns obtained by both a direct and nested approach were similar. However, profi les derived from nested PCR contained more bands, indicative of the increas ed sensitivity of this approach. Analysis of the active biomass by Polyacry lamide Gel Electrophoresis (PAGE) of small molecular weight RNA (5mwRNA) sh owed that a sustained shift in the diversity of the predominant, metabolica lly active species present occurred within two days of the introduction of the protonphore. Biomass production was reduced by 49%, but the total subst rate removal rate was also reduced by 25%. The combined effect was a 30% de crease in the biomass yield. Introduction of the protonphore caused substra te removal efficiency to decrease from a consistent value of 96% to 68.5% w ith considerable variance. This decline in overall process performance was attributed to a surmountable effect arising from the design of apparatus th at resulted in a decrease in the reactor biomass concentration. Although th e specific biomass volume was consistent throughout, decreased sedimentatio n resulted in solids being removed in the final effluent which decreased th e amount of biomass which could be recycled. The catalytic efficiency of th e biomass increased as reflected by a 3.3 fold increase in the specific sub strate uptake rate. (C) 2000 Elsevier Science Ltd. All rights reserved.